1. Field of the Invention
This invention relates generally to an electrochemical reference electrode and, particularly, to a diffusion membrane material useful as a liquid junction structure of a salt bridge tube such as is used in reference electrodes for ion potential measurements of solutions.
2. Description of the Prior Art
In making measurements of the ion concentration of solutions, a reference electrode is commonly employed in conjunction with a sensing electrode, such as a glass electrode, with both electrodes immersed in the test solution. The potential difference between the two electrodes is a function of the concentration of a specific ion in the solution. A typical example is the conventional pH meter and electrode pair used for measuring hydrogen ion concentrations of solutions.
A reference electrode ordinarily comprises an internal half cell supported in a tube containing a salt solution, the tube of salt solution being known as a salt bridge. The salt bridge solution is a strong equitransferent salt solution such as saturated potassium chloride or potassium nitrate. Electrical connection between the salt solution and the sample or test solution is made by liquid flow through a suitably formed aperture or passage in the tube, generally referred to as a liquid junction structure or leak structure. Sometimes the entire unit consisting of the internal half cell structure, the tube, the salt solution and the liquid junction structure is referred to as a half cell; however, for the present specification, the entire unit will be referred to as a reference electrode.
Various means have been utilized for forming the liquid junction structures of salt bridge tubes, including agar gel connections, wicking, asbestos fibers, small capillary tubes, glass tubes with cracks therein, sintered glass plugs sealed in glass tubes, annular passages provided between solid metal rods and the walls of the tubes, porous ceramic rods, porous sintered plastic rods, and ground glass sleeves.
The liquid junction behavior must be substantially independent of the character of the test solution and thus give substantially reproducible potentials. Although the structures referred to above generally satisfy this requirement, they have one or more objectionable features including slow and costly methods of manufacture, high rate of flow of salt solution, lack of uniformity in flow rates and electrical resistance with like liquid junction structures, and lack of durability.
The above mentioned liquid junction structures have two primary disadvantages when utilized in process applications wherein the electrodes must be disposed in high pressure process streams containing viscous liquids. One of these disadvantages is the requirement of an external pressurization of the salt bridge solution in the reference electrode in order to assure that there is a flow of salt bridge solution from the electrode into the sample so that electrical contact will be maintained during the analysis of the sample. Otherwise sample will migrate into the salt bridge solution contaminating the same and also the internal half cell, thus resulting in sparious potentials being developed. The second disadvantage of such liquid junction structures is that they become clogged in many process streams due to viscous substances contained therein, such as molasses, clay slurries, gelatin mixtures, etc., thereby preventing the flow of salt solution from the reference electrode into the sample medium. When this occurs, the electrical connection between the two solutions ceases and the reference electrode is rendered inoperable. What is needed, therefore, is means for providing a liquid junction whose permeability to ions is based essentially upon diffusion, not liquid flow, and thus will not be subject materially to plugging or clogging by viscous liquids or passage of contaminating constituents therethrough from the sample into the salt bridge solution and further eliminating the need for pressurization equipment under normal use of the electrode.
One type of liquid junction structure meeting these requirements employs a naturally hydrophilic semipermeable membrane material, such as cellophane, collodion, cellulose acetate or protein membranes, which closes the open end of a salt bridge tube and is utilized in the analysis of biological fluids such as blood. While these membrane liquid junction structures are not clogged by biological liquids and ions pass therethrough by diffusion rather than liquid flow, they are not entirely satisfactory for process applications since the membranes may deform under varying pressure conditions of the process stream, thus altering the junction potential, and may become ruptured. Thus, what is desired is a liquid junction structure for a reference electrode which has the advantages of the above mentioned hydrophilic membrane liquid junction structures, and in addition is highly durable, has a reasonably low electrical resistance, does not require internal pressurization from an external pressure source and has a reasonably long life, on the order of several months without any attention by an operator. Also, the desired liquid junction structure should be capable of being steam sterilized, have a constant temperature coefficient between like structures and be insensitive to the flow of the sample medium.